CN109085061B - Method for obtaining stress-strain curve of metal material in static compression state - Google Patents
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Abstract
The invention provides a method for obtaining a stress-strain curve of a metal material in a static compression state. The method is a method for improving a compression experiment and processing and analyzing experimental data based on strain control or displacement control, and mainly comprises the following two parts: firstly, experimental data are obtained through an improved compression experiment, and secondly, a real stress-strain curve of the material in an extreme slow compression state, namely a static compression state is obtained through experimental data processing. The method has the advantages that the real stress-strain curve of the material in the long-term compression service process can be obtained through experiments, so that the real use performance of the material can be effectively evaluated, and the method has important guiding significance for the use of the metal material in the long-term compression service state.
Description
Technical Field
The invention relates to the field of metal material performance detection, and relates to a method for acquiring a true stress-strain curve of a metal material in different temperature environments under an extreme slow compression state, namely a static compression state.
Background
Under the service state of a metal material bearing a compression load for a long time, particularly under a high-temperature environment, structural failure is often caused due to the reduction of the strength of the material, so that huge economic loss is caused. Has important guiding significance for the safe service of materials.
Prior to the present invention, metallic materials were subjected to compression experiments, with the experimental loading rate generally maintained at 10-5s-1In the above way, the service condition of long-term stress cannot be effectively simulated, and the faster the compression loading rate is, the higher the detected material performance is, the larger the difference with the real material performance of the material in a state of long-term compression bearing is. The relaxation experiment can measure the real stress condition of a single point, but the relaxation experiment can not obtain a complete material tensile curve, the relaxation experiment consumes a long time, and the experiment time cost is high; meanwhile, particularly for anisotropic materials, the stress-strain curve of the material in a tensile state can not represent the compression performance of the material, so that the method for obtaining the true stress-strain curve of the metal material in a limit slow compression state, namely a static compression state, has important significance.
Disclosure of Invention
The invention aims to provide a method for obtaining a real stress-strain curve of a metal material under different temperature environments in an extreme slow compression state, namely a static compression state. Has important guiding significance for the use of metal materials in a compressed service state for a long time.
In order to achieve the above object, the method for obtaining the true stress-strain curve of the metal material under different temperature environments in the extreme slow compression state, i.e. the static compression state, provided by the invention comprises the following steps:
1. an improved compression experimental method, which can acquire data used for subsequent processing and analysis, specifically comprises the following steps:
1) the experimental conditions are as follows: firstly, the compression tester must have a matched experimental environment temperature box and an extensometer which can be applied to the corresponding environment temperature, and if the compression tester does not have the function, the method can only obtain experimental data at normal temperature; next, the compression testing machine must have a strain control function or a displacement control function, and if the compression testing machine does not have the strain control function or the displacement control function, the test cannot be performed, and it is recommended to perform the test by using a thermal simulation testing machine such as Gleeble 3500.
2) A metallic material compression test specimen is prepared and heated to a predetermined test temperature, and the test temperature is kept constant throughout the test.
3) The compression is carried out under the condition of a certain load rate, when the compression is carried out to the strain value of the material at-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0%, the strain control or displacement control is carried out, the strain is kept for 30 minutes to 2 hours, and all stress, strain and time data in the process are collected.
2. Experimental data processing, the method can obtain the true stress-strain curve of the metal material under different temperature environments in the extreme slow compression state, namely the static compression state, and the specific experimental steps are as follows:
1) a stress-strain curve and a stress-time curve are derived from the experimental data of the previous step.
2) And converting the engineering strain into the real strain under the conventional compression state.
3) And converting the engineering stress into the real stress under the conventional compression state.
4) And respectively carrying out data fitting solution on the time stress curves at-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0% strain positions.
5) Solving according to the fitting to obtain a formula, and solving the stress value sigma corresponding to infinite condition corresponding to times。
6) Corresponding to different strain pointssAnd connecting and drawing true stress-strain curves of the metal material under different temperature environments in an extreme slow compression state, namely a static compression state.
The method has the effect that the actual stress-strain curve of the metal material can be accurately obtained when the metal material bears constant compression in different temperature environments through the method for obtaining the actual stress-strain curve of the metal material in different temperature environments under the extreme slow compression state, namely the static compression state. The material performance curve obtained by the invention has important guiding significance for the use of metal materials in a compressed service state for a long time.
Drawings
FIG. 1 is a plot of engineering compressive stress-strain data directly obtained by a compression tester;
FIG. 2 is a true stress-strain curve calculated;
FIGS. 3, 6, and 9 are time stress curves at-0.5%, -2.0%, -3.5% strain and data plots, respectively;
FIGS. 4, 7, 10 are time strain curves at-0.5%, -2.0%, -3.5% strain, respectively;
FIGS. 5, 8, 11 are time temperature curves at-0.5%, -2.0%, -3.5% strain, respectively;
FIG. 12 is a plot of true stress-strain for extreme slow compression;
FIG. 13 is a comparison of an engineering compressive stress-strain data curve, a true stress-strain curve, and a stress-strain curve at extreme slow compression.
Detailed Description
The method for acquiring the true stress-strain curve of the metal material under different temperature environments in the extreme slow compression (static compression) state is described by combining the attached drawings
The method for acquiring the true stress-strain curve of the metal material under different temperature environments in the extreme slow compression state, namely the static compression state, comprises the following steps of:
1. an improved compression experimental method, which can acquire data used for subsequent processing and analysis, specifically comprises the following steps:
1) the experimental conditions are as follows: firstly, the compression tester must have a matched experimental environment temperature box and an extensometer which can be applied to the corresponding environment temperature, and if the compression tester does not have the function, the method can only obtain experimental data at normal temperature; secondly, the compression tester must have the strain control or displacement control function, and if the compression tester does not have the strain control or displacement control function, the test cannot be carried out.
2) Preparing a metal material compression experiment sample, preferably adopting a round bar sample, shortening the length of the sample as much as possible on the premise of ensuring normal experiment, ensuring the stability of the sample in the compression process, heating the experiment sample to a preset experiment temperature, and keeping the experiment temperature constant in the whole experiment process, as shown in time temperature curves of figures 5, 8 and 11, keeping the temperature difference of the experiment temperature not more than +/-3 ℃ in the whole experiment heat preservation process, if the temperature difference is larger, the experiment result is not recommended to be adopted, and keeping the strain loading rate of the compression experiment at 1.0 × 10-5s-1On the left and right sides, the slower the test loading rate, the more stable the test, and the closer the obtained test result is to the true value.
3) The compression is carried out under the condition of a certain load rate, and when the material is compressed to the strain value of-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0%, the strain control or displacement control is carried out, as shown in time strain curves of fig. 4, fig. 7 and fig. 10, the strain fluctuation is controlled within the range of 0-0.01% in each load-holding time, if the strain fluctuation exceeds the upper limit, the test result is not recommended to be adopted, the strain is kept for 30 minutes to 2 hours, the preferred time is 1 hour, the final stress value is more stable as the time is longer, but the final recommended time is 1 hour as the time cost and the bearing capacity of the experimental equipment are considered, and all the stress, strain and time data in the process are collected.
2. Experimental data processing, the method can obtain the true stress-strain curve of the metal material under different temperature environments in the extreme slow compression state, namely the static compression state, and the specific experimental steps are as follows:
1) a stress-strain curve will be derived from the experimental data of the previous step, as shown in figure 1.
2) And converting the engineering strain into the real strain under the conventional compression state. The situation that the compressive deformation exists in the loading process of the compressive sample (the situation is completely unavoidable), and meanwhile, the stress-strain data acquired by the experimental equipment is data under the premise that the existence of the deformation is not considered, so the data is called as engineering stress-strain data, so the true stress-strain data and the true strain S of the material are obtained through calculationReality (reality)And engineering strain SEngineering ofThe calculation formula of (a) is as follows:
Sreality (reality)＝SEngineering of*(1+σEngineering of)
3) Converting the engineering stress into a true stress, true stress sigmaReality (reality)And engineering stress sigmaEngineering ofThe calculation formula of (a) is as follows:
σreality (reality)＝ln(1+σEngineering of)*100
And performing data drawing on the real stress and the real strain to obtain a real stress-strain curve according to the real stress and the real strain data calculated by the data, as shown in fig. 2.
4) And respectively carrying out data fitting solution on strain time stress curves of-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0%, and the like.
Since the load decreases with time while the displacement or strain of the material is kept constant, and the load decreases with time, the load value eventually approaches a constant small stable value. The blue curve of figure 3 records the time true stress curve at-0.5% strain.
Because the test time cannot be too long due to the consideration of factors such as test time, cost, equipment and the like, the load value under the corresponding time needs to be obtained through calculation, and therefore the data calculation is carried out by adopting the following formula according to the characteristics of the curve:
σreality (reality)＝σFinally, the product is processed+a*exp(-t/b)+c*exp(-t/d)
In the formula sigmaReality (reality)The actual stress value corresponding to each point is obtained through the previous calculation; sigmaFinally, the product is processedThe minimum stable true stress value which is finally approached by the curve is also a constant value which is obtained by data calculation; a. b, c and d are constant values required to be obtained through data calculation; t is the time value inside the corresponding curve.
The data calculation is carried out by utilizing the formula to obtain the following numerical values and formulas:
σfinally, the product is processed＝435.92，a＝141.85，b＝105.30，c＝15.42，d＝993.2。
σReality (reality)＝435.92+141.85exp(-t/105.30)+15.42exp(-t/993.2)
The curve corresponding to the formula is shown in FIG. 3, and the real performance sigma of the material in long-term service under the strain condition is shownFinally, the product is processed＝435.92MPa。
5) Solving according to the fitting to obtain a formula, and solving the stress value sigma corresponding to infinite condition corresponding to times。
According to the formula obtained by the fitting solution in the previous step, the time t is brought into infinity to obtain
σs＝σFinally, the product is processed＝435.92MPa
6) Corresponding to different strain pointssThe connection plots the true stress-strain curves of the metal material under different temperature environments in the extreme slow compression (static compression) state, as shown in fig. 12.
Fig. 13 is a comparison of the engineering compressive stress-strain data curve, the true stress-strain curve, and the stress-strain curve under extreme slow compression shown in fig. 1, 2, and 12, respectively.
Four specific calculation examples are given below.
Example 1
FIG. 6 shows a compression trialThe true stress-strain curve at strain-2.0% is determined by the formulaReality (reality)＝σFinally, the product is processedAnd + a × exp (-t/b) + c × exp (-t/d) performing data fitting on the actual stress strain data obtained by the test to obtain the following results:
σfinally, the product is processed＝474.34，a＝6.39e29，b＝72.33，c＝12365.3，d＝657.71
σReality (reality)＝474.34+6.39e29*exp(-t/72.33)+12365.3*exp(-t/657.71)
The fitting curve is shown in FIG. 6, and the time t is brought to infinity according to the formula obtained by the fitting solution in the previous step to obtain
σs＝σFinally, the product is processed＝474.34MPa
Example 2
FIG. 9 shows the true stress-strain curve of the compressive specimen at 3.5% strain using the formula σReality (reality)＝σFinally, the product is processedAnd + a × exp (-t/b) + c × exp (-t/d) performing data fitting on the actual stress strain data obtained by the test to obtain the following results:
σfinally, the product is processed＝527.54，a＝2.80e32，b＝128.92，c＝6120.9，d＝1402.10
σReality (reality)＝527.54+2.80e32*exp(-t/128.92)+6120.9*exp(-t/1402.10)
The fitting curve is shown in FIG. 9, and the time t is brought to infinity according to the formula obtained by the fitting solution in the previous step to obtain
σs＝σFinally, the product is processed＝527.54MPa。
Claims (7)
1. A method for obtaining a stress-strain curve of a metal material in a static compression state comprises the following steps:
step S1, acquiring processing analysis data by using an improved compression experimental method, specifically including the steps of:
s101, experimental conditions: the compression tester is provided with a matched test environment temperature box and an extensometer corresponding to the environment temperature; the compression tester has the function of strain control or displacement control;
s102, preparing a sample for a metal material compression experiment: heating the experimental sample to a preset experimental temperature, and keeping the experimental temperature constant in the whole experimental process;
s103 compressive load rate requires that the strain loading rate be maintained at 1.0 × 10-4s-1—1.0×10-6s-1To (c) to (d);
step S2, compression test: installing a prepared sample on a compression tester to start stretching, carrying out strain control when the strain value of the material is compressed to-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0%, respectively keeping the strain value for 30-120 minutes and collecting all stress, strain and time data in the process;
step S3, processing experimental data and drawing a stress-strain curve: processing the experimental data of the step S2 and obtaining the real stress-strain curve of the metal material under different temperature environments in a static stretching state, wherein the specific experimental steps are as follows:
s301, deriving and drawing a stress-strain curve and a stress-time curve from the experimental data of the step S2;
s302, converting the engineering strain into a real strain;
s303, converting the engineering stress into a real stress;
s304, respectively carrying out data fitting solution on time stress curves at-0.5%, -1.0%, -1.5%, -2.0%, -2.5%, -3.0%, -3.5%, -4.0% strain positions;
s305, solving according to the fitting to obtain a formula, wherein the solving time corresponds to the stress value sigma corresponding to the infinite conditions；
S306 corresponding sigma of different strain pointssAnd connecting and drawing true stress-strain curves of the metal material under different temperature environments in an extreme slow compression state, namely a static compression state.
2. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: s101 experimental conditions the compression testing machine adopts a Gleeble3500 thermal simulation testing machine.
3. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: step S102, the metal material compression experiment sample is a round rod.
4. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: the retention time in step S2 was 1 hour.
5. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: different temperature environment ranges in step S3: room temperature to 1000 DEG C
6. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: and the length of the S102 sample is less than or equal to the length of the normal test sample.
7. The method for obtaining the stress-strain curve of the metal material under the static compression state as claimed in claim 1, wherein: and S2, displacement control is adopted in the compression test.
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